Polyethylene terephthalate films have heat shrinkability and are good in mechanical and optical properties, and are thus widely used as package films and label films for beverage bottles and the like. However, since polyethylene terephthalate films have high thermal stability, they have a high heat shrinkage initiation temperature and are thus inconvenient in use. Additionally, the heat shrinkability of existing polyethylene terephthalate films is unsatisfactory. Therefore, it is desirable to modify polyethylene terephthalate films so as to lower the heat shrinkage temperature and enhance the heat shrinkability thereof.
U.S. Pat. No. 6,630,230 discloses a polyester resin composition for a polyester film. The polyester film has a heat shrinkage ratio of 15% or more along a main shrinking direction at 70° C. Specifically, the heat shrinkage ratio along the main shrinking direction at 70° C. is in a range from 20% to 22%, as shown in the illustrated examples therein. The polyester resin composition includes an amorphous polyethylene terephthalate resin and a polyester ether resin in admixture at a weight ratio of 97/3 to 50/50. The polyester ether resin is prepared by copolycondensation of dicarboxylic acid and/or ester derivatives thereof, and diols consisting mainly of a C2-C6 alkylene glycol and a poly(C2-C4 alkylene oxide) glycol with a number average molecular weight ranging from 500 to 6000. Segments derived from the poly(C2-C4 alkylene oxide) glycol constitute 1 to 30% by weight of the polyester ether resin. The heat shrinkage ratio of the polyester film at 70° C. is unsatisfactory. Additionally, it is not mentioned therein how deterioration of such physical properties as shrinkability and the like after aging is suppressed.
U.S. Pat. No. 6,663,928 discloses a heat-shrinkage polyester film which has a haze lower than 10% and a heat shrinkage ratio of from 5% to 60% after treatment in hot water at 70° C. for 5 seconds. The heat-shrinkage polyester film is made from a polyester composition which includes a polyester and a polyester elastomer. The polyester has a glass transition temperature of from 60° C. to 75° C. The polyester elastomer is polyester block copolymer composed of high melting point crystalline polyester segments (hard segments) and low melting point soft polymer segments having molecular weights of 400 or higher (soft segments). The melting point of the high molecular weight polymer formed only by the high melting point crystalline polyester segments is 200° C. or higher. Although the heat-shrinkable polyester film disclosed in U.S. Pat. No. 6,663,928 has a heat shrinkage ratio of from 5% to 60% after treatment in hot water at 70° C., it has an inferior haze and the heat shrinkability thereof may be deteriorated after aging.